Optical linear frequency modulation (LFM) signals have many uses in optical devices and processors. For example, optical LFM signals can be used to generate optical signals, to interact with optical signals, and to probe the optical spectral contents of devices or materials.
In a recent approach described in U.S. Pat. No. 7,265,712, by Kristian Doyle Merkel, Zachary Cole, Krishna Mohan Rupavatharam, William Randall Babbitt, Kelvin H. Wagner and Tiejun Chang, entitled “Techniques for Processing High Time-Bandwidth Signals Using a Material with Inhomogeneously Broadened Absorption,” issued Sep. 4, 2007 (hereinafter Merkel), a temporally extended optical LFM signal is used as a probe waveform to generate a readout signal that represents a temporal map of the structure of the spectral population grating (also referred to as spatial-spectral grating or S2 grating) in an inhomogeneously broadened transition (IBT) material, rather than its Fourier transform. This temporal map signal can be measured with inexpensive, high-dynamic-range, megaHertz (MHz, 1 MHz=106 Hertz, 1 Hertz equals one cycle per second) bandwidth detectors and digitizers. Such chirps generally have a duration greater than the decoherence time and less than the population decay time of the inhomogeneously broadened absorption spectrum in IBT material. As described in Merkel, an optical LFM signal sweeping over some wideband portion of the IBT frequency absorption profile of interest, e.g., typically in excess of 1 gigaHertz (GHz, 1 GHz=109 Hertz) can produce a low-bandwidth readout signal that can be detected and digitized with the low-bandwidth high-dynamic-range devices currently available. This low-bandwidth readout signal represents a temporal map of the spectral features in the spatial-spectral grating. For example, in some cases the readout signal includes a temporal spike that represents a single frequency hole burned in the IBT material, and in other cases the readout signal includes a superposition of low-bandwidth beat frequencies, each beat related to a periodic component in the frequency spectrum of the grating.
However, current known techniques for producing spectrally pure, phase continuous radio frequency chirps that are linear in frequency and very stable are limited to pulses with bandwidths less than about 400 MHz. The RF chirp can be impressed on an optical signal using an optical modulator such as an electro-optical modulator (EOM) or an acousto-optic modulator (AOM). Such limited bandwidths are inadequate to make full use of the spectral recording properties of the IBT materials, which extends over tens to hundreds of gigaHertz, and have a wide range of uses.